An Ag thin film is a material which becomes very high in reflectivity with respect to a visible light when the film thickness is about 70 nm or more, and has a low electric resistivity. For this reason, the Ag thin film has conventionally been employed for uses of an optical mirror and the like, or has been studied on its application to an illuminating instrument reflector, or a reflection electrode film of a LED or an organic EL.
A LED reflection electrode film may be exposed to temperatures as high as about 80 to 200° C. according to circumstances. Therefore, by heat, Ag atoms diffuse and agglomerate, so that the surface of the Ag alloy thin film becomes roughened, resulting in a reduced reflectivity. This is a problem in terms of heat resistance. As for the heat resistance, an attempt has been made to attain an improvement by alloying of Ag. For example, there are proposed an alloy film obtained by adding Au to Ag, and further adding Cu or the like thereto, and a conductor film including an alloy film obtained by adding a noble metal to Ag, and an alloy film obtained by adding a rare earth element to Ag, stacked one on another (JP-A No. 2002-129259, JP-A, No. 2006-1271).
Whereas, the LED reflection electrode film has a problem in terms of sulfidation resistance as follows. There is a time period during which the LED reflection electrode film is exposed the air until resin sealing after deposition. During this period, sulfur in the air diffuses and permeates into a protective film such as a resin film or an oxide film. As a result, the LED reflection electrode film may be discolored to black. As for the sulfidation resistance, an attempt has been made to attain an improvement thereof by alloying of Ag. For example, there are proposed an alloy obtained by adding Ge or In to Ag (JP-A No. 2001-192753, JP-A. No. 2006-37169), and an alloy obtained by adding Zn which is an element for improving the sulfidation resistance to an Ag—Bi alloy high in heat resistance (JP-A No. 2005-48231).
Whereas, the Ag film tends to agglomerate, and has a problem of reduction of reflectivity due to agglomeration. The agglomeration is caused by adsorption of halogen ions in the air onto the Ag film surface together with moisture. For this reason, the Ag film is coated on its surface with a resin such as an UV-curable resin or an acrylic resin, an oxide, or a nitride in order to cut off the outside environment for use (JP-A No. 2000-106017, JP-A No. 2006-98856). However, due to permeation of moisture or halogen ions through pin hole portions, and in addition, in the case of a resin coating, due to diffusion and penetration of moisture or halogen ions through the resin itself, agglomeration occurs. Accordingly, a large number of white spots and discoloration occur in the substrate surface on which the Ag film is deposited. This causes reduction of designability and merchantability.